A) Field of Invention
The present invention relates to winding wedge retention, and more particularly, to a coil winding retention scheme within a generator rotor to maintain coil form during high speed rotation.
B) Description of Related Art
Typical wound generator rotors use a center wedge to retain the copper coil (winding) in the rotor slot. The center wedge generally is held in place by one of different techniques. One technique provides for retaining the wedge itself under the pole tips, utilizing several different wedges to accomplish this. A second technique, which is more commonly utilized in designs that have conduction cooling through their so-called cooling wedges, provides for holding the cooling wedges in place by rotor end bands, such as schematically shown in FIG. 1 of the drawings.
Referring to FIG. 1, cooling wedge 12 retains winding (coil) 10 in place and itself is held in place by rotor end bands (not shown). Rotor pole tips 14 do not hold cooling wedge 12 in place in this coil retention scheme. Cooling wedges are discussed in various references including U.S. Pat. No. 4,943,746, which is incorporated herein by reference.
In typical coil retention schemes, impregnation is used to minimize relative motion of the coil to itself as well as the adjoining parts. During relatively low tip speeds and in rotors with small diameters, impregnation in combination with current designs provides for acceptable coil retention. However, in rotors with large diameters (e.g. 12 inch) and/or in high tip speed machines, such as required in certain train applications operating, for example, at 15,000 revolutions per minute, centrifugal loads that are experienced during operation cause the coils and wedges to move outwards radially from the rotor center despite the coil retention schemes. Coil and wedge movement, in turn, disadvantageously results in nonuniform deformation of the coils themselves. As rotor diameter increases, movement and deformation pronouncedly increases.
Coil movement may lead to fretting and/or wear of the coil insulation which would result in electrical failure. Electrical failure damage rotor performance. This phenomenon is cyclical and its frequency is generally dependent on the number of accelerations/decelerations cycles, as well as, the number of thermal cycles.
FIG. 2 is a schematic illustration of the relative positions of coil 10 and cooling wedge 12 during high speed rotation (i.e., a large centrifugal load). As shown in FIG. 2, location 1 identifies an initial point of concern involving the interface of the coil with the cooling wedge. The area identified is affected due to the non-uniform movement of the coil. A portion of the coil losses contact with the cooling wedge resulting in a reduction in heat management which in turn leads to hot spots. Hot spots in turn result in eventual failure of the device. Location 2 depicts the deformation between the coils themselves. The resulting wear on the coils may lead to insulation breakdown and hence reduced life of the rotor.